The ATV does have a DAC, so you can play it through the analogue connections on the back. However, the Sony's D-to-A conversion will almost certainly sound better, so I'd suggest you try the optical out from the ATV to a spare optical in on the Sony.
I don't know (because haven't tried) whether the HDMI output of the ATV carries audio - if it does, then you can use that if your Sony has HDMI capabability. Will check for you.
EDIT - it does, so you have two options - optical or HDMI.